How big are lahars?
Lahars can be of any size. They may be as small as several centimeters wide and deep, flowing less than one meter per second. Steep, unvegetated slopes during a heavy rain are often good sites to observe such small flows. At the other extreme, they can be a few hundred meters wide, tens of meters deep, flow at several tens of meters per second, and travel over 100 kilometers from a volcano. Such catastrophic lahars are triggered by volcanic eruptions or by massive landslides such as the one that occurred on May 18, 1980, at Mount St. Helens volcano. -- From: Brantley and Power, 1985, Reports from the U.S. Geological Survey's Cascades Volcano Observatory at Vancouver, Washington: Earthquake Information Bulletin, v.17, n.1, January-February 1985.